Unite general secretary Steve Turner told the BBC: “Sports Direct workers will be anxious to know what personal details have been hacked in this apparently serious data breach and why they weren’t immediately informed about it by their employer.

“This is potentially sensitive and personal information such as national insurance numbers and bank details that we’re talking about.

“It’s completely unacceptable that the workers affected appear not to have been informed and the data breach swept under the carpet.”

The union has contacted Sports Direct to clarify what happened in the breach, but urged staff to check their financial records, change passwords and report any suspicious activity.

Dr Jamie Greaves, chief executive at cybersecurity company ZoneFox told the BBC: “The way Sports Direct has handled their data breach last year is a perfect example of how not to deal with a cyber-attack.

“Keeping their 30,000-strong workforce in the dark for over a year is simply unacceptable.”

It is not the first time Sports Direct has been criticised for how it treats its staff.

The chairman of the government’s Business, Innovation and Skills committee Iain Wright suggested that Sports Direct’s working practices were “closer to that of a Victorian workhouse than that of a modern, reputable high street retailer”.

The company has also been investigated over staff being paid below the minimum wage.